This section provides background information related to the present disclosure which is not necessarily prior art.
A wide variety of processes involve removing suspended solids from fluid streams. Typical examples include pretreatment systems for reverse osmosis desalination plants, waste water treatment plants and various food processing applications.
One filtration process is called Dissolved Air Flotation (DAF). The DAF process uses fluid saturated with air in a way that can remove most suspended solids from water streams. The DAF process, while effective, consumes large amounts of energy needed to pump water at high pressure to facilitate generation of water saturated with air. In addition, special tanks and controls are needed to regulate the process of dissolving air into the pressurized water stream. However, the DAF process described above may be adapted to use gasses other than air and fluids other than water. Thus, gas is used in place of water in the following description.
FIG. 1 illustrates a typical DAF system 10. The raw water stream enters the DAF system through a primary inlet pipe 12 and flows into an inlet end 13 of a flotation tank 14. A plume of microscopic air bubbles 16 rises through raw water 18 in flotation tank 14. The bubbles 16 stick to suspended solids such as tiny drops of oil, grease or algae cells and cause the particles to float to the surface forming a sludge layer 20. A moving conveyor skimmer 22 drags the floating sludge layer 20 to a hopper 24.
A coagulant may be added from a coagulant source 26 to the raw water stream in the primary inlet pipe 12 to promote agglomeration of particles to increase the effectiveness of the floatation process. Heavy materials 30 may settle to the bottom of flotation tank 14. The heavy materials 30 are collected by heavy material hopper 32. Relatively clean water may be discharged through a discharge outlet 34 at an outlet end 36 of the filtration tank 14.
A partition 40 near the outlet end 36 of the flotation tank 14 forms an area of relatively clean water near the outlet end 36 of the partition 40. A secondary outlet 44 draws water from the outlet end 36 of the partition 40 near the outlet end 36 and fluidically communicates the relatively clean water to pump 46. The pump 46 typically pressurizes the relatively clean water to 80 to 100 pounds per square inch gauge (psig). The pressurized relatively clean water enters an absorption tank 48 at a water inlet 50. The relatively clean (and pressure increased) water is exposed to pressurized air (at the same pressure as the water) within the absorption tank 48. The pressurized air is rapidly absorbed by the water to form a solution at or near the saturation point with air.
The solution exits absorption tank 48 by solution outlet pipe 52. The solution passes through a pressure reduction valve 54 into the solution inlet 56 of the filtration tank 14. The reduction in pressure within the pressure reduction valve 54 results in much of the dissolved air coming out of solution in the form microscopic bubbles, which are directed into flotation tank 14 forming a plume of rising bubbles 16.
Compressed air is supplied to absorption tank 48 by compressor 60 through flow regulating valve 62 and through an air inlet 64 of the absorption tank 48. A fluid level sensor 66 monitors water level in absorption tank 48 and sends a level signal to a controller 70. The controller 70 may communicate a control signal to air regulating valve 64 and/or pump 46 as necessary to maintain proper water level in tank. The pump 46 uses a lot of energy in the process.